The use of softening agents in the manufacture of tissues, such as facial and bath tissue, is common practice in the industry. These tissues typically contain a blend of relatively long fibers, which are usually softwood fibers, and relatively short fibers, which are usually hardwood fibers. Commonly, the softening agent is added to the short fibers prior to forming the web since the short fibers primarily contribute to tissue softness. The long fibers are separately treated with strengthening agents (wet and dry) and refining. Both refining and strengthening agents are often used because excessive use of either treatment may have an adverse effect on the tissue making process and/or the resulting tissue product.
However, the conventional method of adding softening agents to the fiber furnish can have some disadvantages. In one case, softening agents added to a furnish prior to forming cause internal debonding which results in loss of tensile strength and increased dust when the sheet contacts the creping doctor blade. The loss in tensile strength may require excessive refining or strength additive which can have a detrimental effect on tissue softness. Excessive dust may increase the frequency of sheet breaks at the reel, provide a less than desirable environment for machine operators and is detrimental to the consumer perception of the tissue.
Therefore there is a need for a more efficient method of utilizing softening agents in the manufacture of tissues.
It has now been discovered that an especially soft creped tissue can be produced by the indirect addition of chemical softening agents to the tissue web by applying the chemical softening agents to the surface of the Yankee dryer, such as by spraying. More specifically, the softening agents can be included as part of the creping adhesive formulation, which is sprayed onto the surface of the Yankee dryer between the creping blade and the pressure roll. The softening agents are subsequently transferred to the tissue sheet surface as the sheet is pressed against the Yankee dryer.
The softening agent remains predominantly on the surface of the sheet where it is most beneficial, thereby resulting in a slicker, more lotiony surface feel desired by consumers. Some of the softening agent passes through the sheet while in the pressure roll nip and is recirculated back to the wet end of the tissue machine with the white water. As such, additional amounts becomes incorporated into the tissue sheet at that point of the process, but these amounts are more evenly distributed throughout the sheet. The net result is a tissue sheet having a greater concentration of softening agent on the surface of the sheet, thereby improving the surface softness of the sheet relative to sheets made in a conventional manner.
Hence in one aspect the invention resides in a method for making creped tissue comprising: (a) forming a wet tissue web by depositing an aqueous papermaking furnish onto a forming fabric; (b) partially dewatering the tissue web; (c) applying a creping adhesive and one or more softening agents to the surface of a Yankee dryer; (d) adhering the tissue web to the surface of the Yankee dryer such that the softening agent is transferred to the tissue web; and (e) creping the web.
In another aspect, the invention resides in a tissue product made by the abovementioned method.
Suitable softening agents used as part of the adhesive formulation include a range of chemistries that contribute a soft, silky, smooth, velvety, fluffy, lotiony, cushiony, quilted, delicate, satiny, and soothing feel to the tissue. These agents include, but are not limited to: imidazoline quaternaries; ester quaternaries; phospholipids; silicone phospholipids; silicone quaternaries; quaternized lanolin derivatives; hydrolyzed wheat protein/polydimethyl siloxane; hydrolyzed wheat protein/dimethicone phosphocopolyol copolymer; organoreactive polysiloxanes; nonionic surfactants, such as alkylphenol ethoxylates, aliphatic alcohol ethoxylates, fatty acid alkoxylates, fatty alcohol alkoxylates, and block copolymers of ethylene oxide and propylene oxide; condensation products of ethylene oxide with the product resulting from the reaction of propylene oxide and ethylenediamine; condensation products of propylene oxide with the product of the reaction of ethylene oxide and ethylenediamine; semipolar nonionic surfactants, such as water soluble amine oxides; alkylpolysaccharides, such as alkylpolyglycosides; fatty acid amide surfactants; polyhydroxy compounds, including glycerol, polyethylene glycols, and polypropylene glycols having a weight average molecular weight from 200 to 4000; quatemized protein compounds; silicone emulsions and silicone glycols.
The amount of softening agent added to the Yankee dryer can be any amount that is effective in increasing the softness of the resulting tissue and will depend on the particular softening agent selected and the desired softness effect. Nevertheless, suitable amounts of softening agent added to the Yankee dryer, expressed as a weight percent solids based on the dry weight of fiber in the tissue, can be 0.05 weight percent or greater, more specifically from 0.05 to about 0.4 weight percent, still more specifically from about 0.1 to about 0.3 weight percent, and still more specifically from about 0.1 to about 0.2 weight percent.